I am trying to measure the noise of a photo detector at different frequencies, using an RF spectrum analyzer. The measurement range was spanning from 10 Hz to 20 MHz. To cover not only the higher frequencies with a sufficient amount of points, but also the lower range, I repeated the measurement several times while varying the span (10 Hz to 20 MHz, 1 MHz, 100 kHz and 10 kHz), but keeping the amount of points constant. Initially, the instrument set the sweep time automatically, resulting in the following graph: Here the transitions are clearly visible at places where the noise level decreases.
To avoid such rapid transitions, I tried to keep the sweep time constant. This resulted in Here for all measurements (except the one going up to 20 MHz) the sweep time was fixed, but I still see transitions between the measurements.
Where do those transitions come from, and how can I avoid them in future measurements?